It's a comet that, as far as anyone knows, has never passed through the solar system before. And it is a sun-grazer. At its closest, it will be close -- about 800,000 miles from the sun. That's what makes astronomers believe it could be so bright.

A comet is either a dirty ice ball or an icy dirt ball. Coming from outside the solar system, they tend to melt as they approach the sun.

The trail of dust and gas they leave behind creates the comet's tail. If the tail is big enough, people can see it.

As comet-watching goes, we've been lucky. In 1996, we had Comet Hyakutake and in 1997 the wonderful Comet Hale-Bopp -- both spectacular naked-eye objects that hung in the night sky for weeks.

(I have a fondness for Hyakutake. When I used to get home from work late on clear nights, it would be hanging in the sky over a vegetable garden I had in my yard. It was like a friend, stopping in for a visit.)

Comets can be periodic. They come around regularly and can be at least a little predictable.

But Chester said because no one has seen ISON before, no one knows how it will behave or what it will look like.

Sometimes, comets fall apart as they approach the sun and never get as bright as astronomers hoped for. That's what happened to Kohoutek.

But Chester said that in 1976 Comet West broke into four fragments. Because a haze of gas and dust surrounded each fragment, it grew bigger and brighter that anyone expected. Maybe, because of the public's disenchantment with Kohoutek, it was also somewhat ignored.

So here are the ways ISON could develop, he added. It could stay intact and shine brilliantly, like a pale ghost in the sky. It could fall apart and be a dud like Kohoutek. It could fall apart and put on a show like Comet West.

Or, he said, it could be a very nice, naked-eye object that people can see in the early morning eastern sky in late November and December, especially if you're in an unlit place, away from a city's light pollution.

"It may be bright enough to see from urban settings," Chester said.

The problem is this, he said: Predicting a comet's appearance five months before it arrives is like predicting November's weather now. There's a lot of room for error, so it may be wise to acknowledge you don't really know.

"I hope I'm wrong," he said. "But I'd be very surprised if it was the comet of the century. Don't plan on it signaling the end of the world."